The loss of skeletal muscle mass with aging (sarcopenia) contributes to physical frailty. Because skeletal muscle is post-mitotic tissue with a high dependance on oxidative energy metabolism, the accrual of oxidative damage with age may cause reduction of muscle mass and fiber size/number. Our PO1-supported studies of the quadriceps from transport system (ETS) abnormalities and fiber atrophy. Our work in rat skeletal muscles point toward similar associations and shows that dietary restriction (DR) started in late middle-age attenuates these changes in old age. We hypothesize that age-associated muscle fiber atrophy and loss is caused by mtDNA deletions and subsequent ETS dysfunction, leading to oxidatively damaged muscle fibers which atrophy. Further, DR opposes these changes. To test this, we will study vastus lateralis biopsies from adult (approximately 10 years), late middle-age (approximately 20-25 years) and adult (greater than 25 years) conventionally-fed monkeys and others subjected to DR or Control feeding (approximately 25-30 years). Because the age-associated accrual of ETS abnormalities, mtDNA deletions and fiber loss all occur focally, histological approaches are emphasized. Specific Aim 1 is to estimate muscle mass and to characterize the mtDNA genotype and ETS phenotype along fibers showing ETS abnormalities and to determine the fate of these fibers. In samples from vastus lateralis, which is known to undergo sarcopenia, fibers will be examined cross-sectionally along their length for ETS activities and presence of mtDNA deletions (in situ hybridization). Determination of fiber area and cell death assays will provide data on fiber fate. Specific Aim 2 is to determine the extent of oxidative damage in these same fibers and in associated neuromuscular functions using antibodies specific for three selected oxidative damage molecules. This will be done at light and ultrastructural levels to identify affected cell types and subcellular organelles. In Specific Aim 3, using biopsy samples from Grp.1 and NIA monkeys, these methods will be used to examine the influence of DR on the hypothesized pathogenesis of muscle fiber atrophy. By testing this hypothesis, these studies should elucidate the contributions and sequential involvement of oxidative stress, mtDNA deletions, and ETS defects in fiber atrophy/loss and the sensitivity of these processes to DR.